Data transmission method and device in wireless communication system

ABSTRACT

A data transmission method of a station (STA) device in a wireless LAN system is disclosed. The data transmission method of an STA device, according to the present invention, comprises: a receiving step of receiving an uplink data frame from a first STA, wherein the uplink data frame includes first receiving operating mode information indicating a receiving operating mode to be changed by the first STA; and a transmitting step of transmitting an ACK frame for the uplink data frame, wherein the ACK frame includes mode change acceptance information indicating whether to accept or reject the change in the receiving operating mode according to the receiving operating mode information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/012262, filed on Nov. 16, 2015,which claims the benefit of U.S. Provisional Application No. 62/207,936,filed Aug. 21, 2015, the contents of which are hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more particularly, to a station (STA) device for sending a signal byadding a padding symbol so that a receiver can secure a signalprocessing time depending on an increase in the FFT size of transmitteddata in a wireless local area network (WLAN) communication system and amethod for sending, by the STA device, data.

BACKGROUND ART

Wi-Fi is a wireless local area network (WLAN) technology which enables adevice to access the Internet in a frequency band of 2.4 GHz, 5 GHz or60 GHz.

A WLAN is based on the institute of electrical and electronic engineers(IEEE) 802.11 standard. The wireless next generation standing committee(WNG SC) of IEEE 802.11 is an ad-hoc committee which is worried aboutthe next-generation wireless local area network (WLAN) in the medium tolonger term.

IEEE 802.11n has an object of increasing the speed and reliability of anetwork and extending the coverage of a wireless network. Morespecifically, IEEE 802.11n supports a high throughput (HT) providing amaximum data rate of 600 Mbps. Furthermore, in order to minimize atransfer error and to optimize a data rate, IEEE 802.11n is based on amultiple inputs and multiple outputs (MIMO) technology in which multipleantennas are used at both ends of a transmission unit and a receptionunit.

As the spread of a WLAN is activated and applications using the WLAN arediversified, in the next-generation WLAN system supporting a very highthroughput (VHT), IEEE 802.11ac has been newly enacted as the nextversion of an IEEE 802.11n WLAN system. IEEE 802.11ac supports a datarate of 1 Gbps or more through 80 MHz bandwidth transmission and/orhigher bandwidth transmission (e.g., 160 MHz), and chiefly operates in a5 GHz band.

Recently, a need for a new WLAN system for supporting a higherthroughput than a data rate supported by IEEE 802.11ac comes to thefore.

The scope of IEEE 802.11ax chiefly discussed in the next-generation WLANtask group called a so-called IEEE 802.11ax or high efficiency (HEW)WLAN includes 1) the improvement of an 802.11 physical (PHY) layer andmedium access control (MAC) layer in bands of 2.4 GHz, 5 GHz, etc., 2)the improvement of spectrum efficiency and area throughput, 3) theimprovement of performance in actual indoor and outdoor environments,such as an environment in which an interference source is present, adense heterogeneous network environment, and an environment in which ahigh user load is present and so on.

A scenario chiefly taken into consideration in IEEE 802.11ax is a denseenvironment in which many access points (APs) and many stations (STAs)are present. In IEEE 802.11ax, the improvement of spectrum efficiencyand area throughput is discussed in such a situation. More specifically,there is an interest in the improvement of substantial performance inoutdoor environments not greatly taken into consideration in existingWLANs in addition to indoor environments.

In IEEE 802.11ax, there is a great interest in scenarios, such aswireless offices, smart homes, stadiums, hotspots, andbuildings/apartments. The improvement of system performance in a denseenvironment in which many APs and many STAs are present is discussedbased on the corresponding scenarios.

In the future, it is expected in IEEE 802.11ax that the improvement ofsystem performance in an overlapping basic service set (OBSS)environment, the improvement of an outdoor environment, cellularoffloading, and so on rather than single link performance improvement ina single basic service set (BSS) will be actively discussed. Thedirectivity of such IEEE 802.11ax means that the next-generation WLANwill have a technical scope gradually similar to that of mobilecommunication. Recently, when considering a situation in which mobilecommunication and a WLAN technology are discussed together in smallcells and direct-to-direct (D2D) communication coverage, it is expectedthat the technological and business convergence of the next-generationWLAN based on IEEE 802.11ax and mobile communication will be furtheractivated.

DISCLOSURE Technical Problem

As described above, a method for improving performance in the 802.11axsystem, that is, the next-generation wireless LAN system, is activelydiscussed. More specifically, a method for improving resources useefficiency in a limited bandwidth is an important subject in the802.11ax system.

In the 802.11ax system, for robust transmission in average throughputenhancement and outdoors, a symbol length four times longer than that inthe legacy 802.11 system (e.g., 802.11a, 802.11n, and 802.11ac) is to beused. That is, when OFDM modulation is performed, an STA may use thefour-times greater FFT size.

In order to enhance system throughput, an STA needs to change receive(Rx) operating mode while sending data. Furthermore, a case where an STAchanges transmit (Tx) operating mode for a target communication STA mayalso be taken into consideration.

Technical Solution

Embodiments of the present invention propose an STA device in a WLANsystem and a method for sending, by the STA device, data.

In an embodiment of the present invention, a method for transmitting, bya station (STA), data in a wireless LAN (WLAN) system includes receivingan uplink data frame from a first STA, the uplink data frame includingfirst receive (Rx) operating mode information indicative of Rx operatingmode to be changed by the first STA and sending an ACK frame for theuplink data frame, the ACK frame including mode change acceptanceinformation indicative of an acceptance or denial of an Rx operatingmode change according to the Rx operating mode information. The first Rxoperating mode information includes received spatial stream numberinformation and Rx channel bandwidth information.

In the method for sending, by an STA, data according to an embodiment ofthe present invention, the ACK frame may further include second Rxoperating mode information for the first STA if the mode changeacceptance information is indicative of the denial of the Rx operatingmode change. The second Rx operating mode information may be indicativeof Rx operating mode different from the first Rx operating mode.

In the method for sending, by an STA, data according to an embodiment ofthe present invention, the uplink data frame may include Tx operatingmode information. The Tx operating mode information may be indicative ofTx operating mode of the STA which is to be received by the first STA.

In the method for sending, by an STA, data according to an embodiment ofthe present invention, the Tx operating mode information may includeresource unit information indicative of a resource unit size by whichthe first STA is capable of accessing the STA.

The method for sending, by an STA, data according to an embodiment ofthe present invention may further include sending trigger informationindicative of the transmission of the Rx operating mode information.

The method for sending, by an STA, data according to an embodiment ofthe present invention may further include initiating data transmissionaccording to changed operating mode after the ACK frame is transmittedif the operating mode change according to the Rx operating modeinformation is accepted. The data transmission may be initiated after adelay time from the transmission of the ACK frame.

In the method for sending, by an STA, data according to an embodiment ofthe present invention, the ACK frame may include delay time informationindicative of data transmission duration according to operating modeprior to a change after the ACK frame is transmitted.

Furthermore, a station (STA) device in a wireless LAN (WLAN) systemaccording to an embodiment of the present includes a radio frequency(RF) unit transmitting and receiving a radio signal and a processorcontrolling the RF unit. The STA device receives an uplink data framefrom a first STA and sends an ACK frame for the uplink data frame. Teuplink data frame includes first receive (Rx) operating mode informationindicative of Rx operating mode to be changed by the first STA. The ACKframe includes mode change acceptance information indicative of anacceptance or denial of an Rx operating mode change according to the Rxoperating mode information. The first Rx operating mode informationincludes received spatial stream number information and Rx channelbandwidth information.

Advantageous Effects

An STA according to an embodiment of the present invention can activelychange the number of reception chains for power saving depending on thenumber of received spatial streams and a channel bandwidth whenperforming spatial multiplexing. More specifically, the STA can changeRx operating mode without a separate procedure, such as the transmissionand reception of an additional frame, by sending a transmission dataframe including Rx operating mode information to be changed.

Furthermore, an STA that has received a signal frame including Rxoperating mode information may send information indicative of theacceptance or denial of an operating mode change using an ACK frameother than a separate frame. Furthermore, if an STA denies an operatingmode change, it may additionally transmit information about anotheroperating mode to be changed by the STA using an ACK frame.

Accordingly, in accordance with an embodiment of the present invention,system throughput can be enhanced because Rx operating mode of an STAcan be changed through the transmission and reception of a common dataframe and an ACK frame without a separate additional procedure.

Furthermore, if an STA is capable of communication with another STA (AP)using a specific resource unit size, it can communicate with the APhaving wide coverage, such as an outdoors environment, by sending theresource unit size to another STA.

The Rx operating mode information may be transmitted, if necessary, orthe transmission of the Rx operating mode information may be initiatedby a trigger signal in order to enhance system throughput.

Furthermore, an STA that receives Rx operating mode information andchanges operating mode can perform data transmission according tochanged operating mode after a specific outage time, thereby beingcapable of preventing an error in the reception of a receiver.Furthermore, the flexibility of a system operation can be achieved bysending data in existing operating mode during a specific outage timeand sending data in changed operating mode after the outage time, ifnecessary.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichan embodiment of the present invention may be applied.

FIG. 2 is a diagram illustrating the configuration of layer architectureof an IEEE 802.11 system to which an embodiment of the present inventionmay be applied.

FIG. 3 illustrating a non-HT format PPDU and an HT format PPDU in awireless communication system to which an embodiment of the presentinvention may be applied.

FIG. 4 illustrates a VHT format PPDU in a wireless communication systemto which an embodiment of the present invention may be applied.

FIG. 5 illustrates the format of an MAC frame for an IEEE 802.11 systemto which an embodiment of the present invention may be applied.

FIG. 6 is a diagram illustrating a frame control field within an MACframe in a wireless communication system to which an embodiment of thepresent invention may be applied.

FIG. 7 illustrates the VHT format of an HT control field in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 8 is a diagram illustrating a random backoff period and a frametransmission procedure in a wireless communication system to which anembodiment of the present invention may be applied.

FIG. 9 is a diagram illustrating an IFS relation in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

FIG. 10 is a diagram illustrating a downlink MU-MIMO transmissionprocess in a wireless communication system to which an embodiment of thepresent invention may be applied.

FIG. 11 shows an HE PPDU format according to an embodiment of thepresent invention.

FIG. 12 shows an HE-PPDU format according to another embodiment of thepresent invention.

FIG. 13 shows an HE-PPDU format according to another embodiment of thepresent invention.

FIG. 14 shows the UL MU data transmission of an HE system according toan embodiment of the present invention.

FIG. 15 shows the UL MU data transmission of an HE system according toan embodiment of the present invention.

FIGS. 16 to 18 show the transmission of information by an STA in Rxoperating mode according to an embodiment of the present invention.

FIG. 19 shows a multi-STA BA frame format according to an embodiment ofthe present invention.

FIG. 20 shows the transmission of Rx operating mode information and amethod for sending data in Rx operating mode according to an embodimentof the present invention.

FIG. 21 shows an STA device according to an embodiment of the presentinvention.

FIG. 22 shows a method for sending, by an STA device, data according toan embodiment of the present invention.

MODE FOR INVENTION

Hereinafter, some embodiments of the present invention are described indetail with reference to the accompanying drawings. The detaileddescription to be disclosed herein along with the accompanying drawingsis provided to describe exemplary embodiments of the present inventionand is not intended to describe a sole embodiment in which the presentinvention may be implemented. The following detailed descriptionincludes detailed contents in order to provide complete understanding ofthe present invention. However, those skilled in the art will appreciatethat the present invention may be implemented even without such detailedcontents.

In some cases, in order to avoid making the concept of the presentinvention vague, the known structure and/or device may be omitted or maybe illustrated in the form of a block diagram based on the core functionof each structure and/or device.

Furthermore, specific terms used in the following description areprovided to help understanding of the present invention, and suchspecific terms may be changed into other forms without departing fromthe technical spirit of the present invention.

The following technologies may be used in a variety of wirelesscommunication systems, such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and non-orthogonalmultiple access (NOMA). CDMA may be implemented using a radiotechnology, such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be implemented using a radio technology, such asglobal system for Mobile communications (GSM)/general packet radioservice (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMA maybe implemented using a radio technology, such as institute of electricaland electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX),IEEE 802.20, or evolved UTRA (E-UTRA). UTRA is part of a universalmobile telecommunications system (UMTS). 3^(rd) generation partnershipproject (3GPP) long term evolution (LTE) is part of an evolved UMTS(E-UMTS) using evolved UMTS terrestrial radio access (E-UTRA), and itadopts OFDMA in downlink and adopts SC-FDMA in uplink. LTE-advanced(LTE-A) is the evolution of 3GPP LTE.

Embodiments of the present invention may be supported by the standarddocuments disclosed in at least one of IEEE 802, 3GPP, and 3GPP2, thatis, radio access systems. That is, steps or portions that belong to theembodiments of the present invention and that are not described in orderto clearly expose the technical spirit of the present invention may besupported by the documents. Furthermore, all terms disclosed in thisdocument may be described by the standard documents.

In order to more clarify a description, IEEE 802.11 is mainly described,but the technical characteristics of the present invention are notlimited thereto.

General System

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichan embodiment of the present invention may be applied.

The IEEE 802.11 configuration may include a plurality of elements. Theremay be provided a wireless communication system supporting transparentstation (STA) mobility for a higher layer through an interaction betweenthe elements. A basic service set (BSS) may correspond to a basicconfiguration block in an IEEE 802.11 system.

FIG. 1 illustrates that three BSSs BSS 1 to BSS 3 are present and twoSTAs (e.g., an STA 1 and an STA 2 are included in the BSS 1, an STA 3and an STA 4 are included in the BSS 2, and an STA 5 and an STA 6 areincluded in the BSS 3) are included as the members of each BSS.

In FIG. 1, an ellipse indicative of a BSS may be interpreted as beingindicative of a coverage area in which STAs included in thecorresponding BSS maintain communication. Such an area may be called abasic service area (BSA). When an STA moves outside the BSA, it isunable to directly communicate with other STAs within the correspondingBSA.

In the IEEE 802.11 system, the most basic type of a BSS is anindependent a BSS (IBSS). For example, an IBSS may have a minimum formincluding only two STAs. Furthermore, the BSS 3 of FIG. 1 which is thesimplest form and from which other elements have been omitted maycorrespond to a representative example of the IBSS. Such a configurationmay be possible if STAs can directly communicate with each other.Furthermore, a LAN of such a form is not previously planned andconfigured, but may be configured when it is necessary. This may also becalled an ad-hoc network.

When an STA is powered off or on or an STA enters into or exits from aBSS area, the membership of the STA in the BSS may be dynamicallychanged. In order to become a member of a BSS, an STA may join the BSSusing a synchronization process. In order to access all of services in aBSS-based configuration, an STA needs to be associated with the BSS.Such association may be dynamically configured, and may include the useof a distribution system service (DSS).

In an 802.11 system, the distance of a direct STA-to-STA may beconstrained by physical layer (PHY) performance In any case, the limitof such a distance may be sufficient, but communication between STAs ina longer distance may be required, if necessary. In order to supportextended coverage, a distribution system (DS) may be configured.

The DS means a configuration in which BSSs are interconnected. Morespecifically, a BSS may be present as an element of an extended form ofa network including a plurality of BSSs instead of an independent BSS asin FIG. 1.

The DS is a logical concept and may be specified by the characteristicsof a distribution system medium (DSM). In the IEEE 802.11 standard, awireless medium (WM) and a distribution system medium (DSM) arelogically divided. Each logical medium is used for a different purposeand used by a different element. In the definition of the IEEE 802.11standard, such media are not limited to the same one and are also notlimited to different ones. The flexibility of the configuration (i.e., aDS configuration or another network configuration) of an IEEE 802.11system may be described in that a plurality of media is logicallydifferent as described above. That is, an IEEE 802.11 systemconfiguration may be implemented in various ways, and a correspondingsystem configuration may be independently specified by the physicalcharacteristics of each implementation example.

The DS can support a mobile device by providing the seamless integrationof a plurality of BSSs and providing logical services required to handlean address to a destination.

An AP means an entity which enables access to a DS through a WM withrespect to associated STAs and has the STA functionality. The movementof data between a BSS and the DS can be performed through an AP. Forexample, each of the STA 2 and the STA 3 of FIG. 1 has the functionalityof an STA and provides a function which enables associated STAs (e.g.,the STA 1 and the STA 4) to access the DS. Furthermore, all of APsbasically correspond to an STA, and thus all of the APs are entitiescapable of being addressed. An address used by an AP for communicationon a WM and an address used by an AP for communication on a DSM may notneed to be necessarily the same.

Data transmitted from one of STAs, associated with an AP, to the STAaddress of the AP may be always received by an uncontrolled port andprocessed by an IEEE 802.1X port access entity. Furthermore, when acontrolled port is authenticated, transmission data (or frame) may bedelivered to a DS.

A wireless network having an arbitrary size and complexity may include aDS and BSSs. In an IEEE 802.11 system, a network of such a method iscalled an extended service set (ESS) network. The ESS may correspond toa set of BSSs connected to a single DS. However, the ESS does notinclude a DS. The ESS network is characterized in that it looks like anIBSS network in a logical link control (LLC) layer. STAs included in theESS may communicate with each other. Mobile STAs may move from one BSSto the other BSS (within the same ESS) in a manner transparent to theLLC layer.

In an IEEE 802.11 system, the relative physical positions of BSSs inFIG. 1 are not assumed, and the following forms are all possible.

More specifically, BSSs may partially overlap, which is a form commonlyused to provide consecutive coverage. Furthermore, BSSs may not bephysically connected, and logically there is no limit to the distancebetween BSSs. Furthermore, BSSs may be placed in the same positionphysically and may be used to provide redundancy. Furthermore, one (orone or more) IBSS or ESS networks may be physically present in the samespace as one or more ESS networks. This may correspond to an ESS networkform if an ad-hoc network operates at the position in which an ESSnetwork is present, if IEEE 802.11 networks that physically overlap areconfigured by different organizations, or if two or more differentaccess and security policies are required at the same position.

In a WLAN system, an STA is an apparatus operating in accordance withthe medium access control (MAC)/PHY regulations of IEEE 802.11. An STAmay include an AP STA and a non-AP STA unless the functionality of theSTA is not individually different from that of an AP. In this case,assuming that communication is performed between an STA and an AP, theSTA may be interpreted as being a non-AP STA. In the example of FIG. 1,the STA 1, the STA 4, the STA 5, and the STA 6 correspond to non-APSTAs, and the STA 2 and the STA 3 correspond to AP STAs.

A non-AP STA corresponds to an apparatus directly handled by a user,such as a laptop computer or a mobile phone. In the followingdescription, a non-AP STA may also be called a wireless device, aterminal, user equipment (UE), a mobile station (MS), a mobile terminal,a wireless terminal, a wireless transmit/receive unit (WTRU), a networkinterface device, a machine-type communication (MTC) device, amachine-to-machine (M2M) device or the like.

Furthermore, an AP is a concept corresponding to a base station (BS), anode-B, an evolved Node-B (eNB), a base transceiver system (BTS), afemto BS or the like in other wireless communication fields.

Hereinafter, in this specification, downlink (DL) means communicationfrom an AP to a non-AP STA. Uplink (UL) means communication from anon-AP STA to an AP. In DL, a transmitter may be part of an AP, and areceiver may be part of a non-AP STA. In UL, a transmitter may be partof a non-AP STA, and a receiver may be part of an AP.

FIG. 2 is a diagram illustrating the configuration of layer architectureof an IEEE 802.11 system to which an embodiment of the present inventionmay be applied.

Referring to FIG. 2, the layer architecture of the IEEE 802.11 systemmay include an MAC sublayer and a PHY sublayer.

The PHY sublayer may be divided into a physical layer convergenceprocedure (PLCP) entity and a physical medium dependent (PMD) entity. Inthis case, the PLCP entity functions to connect the MAC sublayer and adata frame, and the PMD entity functions to wirelessly transmit andreceive data to and from two or more STAs.

The MAC sublayer and the PHY sublayer may include respective managemententities, which may be referred to as an MAC sublayer management entity(MLME) and a PHY sublayer management entity (PLME), respectively. Themanagement entities provide a layer management service interface throughthe operation of a layer management function. The MLME is connected tothe PLME and may perform the management operation of the MAC sublayer.Likewise, the PLME is also connected to the MLME and may perform themanagement operation of the PHY sublayer.

In order to provide a precise MAC operation, a station management entity(SME) may be present in each STA. The SME is a management entityindependent of each layer, and collects layer-based state informationfrom the MLME and the PLME or sets the values of layer-specificparameters. The SME may perform such a function instead of common systemmanagement entities and may implement a standard management protocol.

The MLME, the PLME, and the SME may interact with each other usingvarious methods based on primitives. More specifically, anXX-GET.request primitive is used to request the value of a managementinformation base (MIB) attribute. An XX-GET.confirm primitive returnsthe value of a corresponding MIB attribute if the state is “SUCCESS”,and indicates an error in the state field and returns the value in othercases. An XX-SET.request primitive is used to make a request so that adesignated MIB attribute is set as a given value. If an MIB attributemeans a specific operation, such a request requests the execution of thespecific operation. Furthermore, an XX-SET.confirm primitive means thata designated MIB attribute has been set as a requested value if thestate is “SUCCESS.” In other cases, the XX-SET.confirm primitiveindicates that the state field is an error situation. If an MIBattribute means a specific operation, the primitive may confirm that acorresponding operation has been performed.

An operation in each sublayer is described in brief as follows.

The MAC sublayer generates one or more MAC protocol data units (MPDUs)by attaching an MAC header and a frame check sequence (FCS) to a MACservice data unit (MSDU) received from a higher layer (e.g., an LLClayer) or the fragment of the MSDU. The generated MPDU is delivered tothe PHY sublayer.

If an aggregated MSDU (A-MSDU) scheme is used, a plurality of MSDUs maybe aggregated into a single aggregated MSDU (A-MSDU). The MSDUaggregation operation may be performed in an MAC higher layer. TheA-MSDU is delivered to the PHY sublayer as a single MPDU (if it is notfragmented).

The PHY sublayer generates a physical protocol data unit (PPDU) byattaching an additional field, including information for a PHYtransceiver, to a physical service data unit (PSDU) received from theMAC sublayer. The PPDU is transmitted through a wireless medium.

The PSDU has been received by the PHY sublayer from the MAC sublayer,and the MPDU has been transmitted from the MAC sublayer to the PHYsublayer. Accordingly, the PSDU is substantially the same as the MPDU.

If an aggregated MPDU (A-MPDU) scheme is used, a plurality of MPDUs (inthis case, each MPDU may carry an A-MSDU) may be aggregated in a singleA-MPDU. The MPDU aggregation operation may be performed in an MAC lowerlayer. The A-MPDU may include an aggregation of various types of MPDUs(e.g., QoS data, acknowledge (ACK), and a block ACK (BlockAck)). The PHYsublayer receives an A-MPDU, that is, a single PSDU, from the MACsublayer. That is, the PSDU includes a plurality of MPDUs.

Accordingly, the A-MPDU is transmitted through a wireless medium withina single PPDU.

Physical Protocol Data Unit (PPDU) Format

A PPDU means a data block generated in the physical layer. A PPDU formatis described below based on an IEEE 802.11 a WLAN system to which anembodiment of the present invention may be applied.

FIG. 3 illustrating a non-HT format PPDU and an HT format PPDU in awireless communication system to which an embodiment of the presentinvention may be applied.

FIG. 3(a) illustrates a non-HT format PPDU for supporting IEEE 802.11a/gsystems. The non-HT PPDU may also be called a legacy PPDU.

Referring to FIG. 3(a), the non-HT format PPDU is configured to includea legacy format preamble, including a legacy (or non-HT) short trainingfield (L-STF), a legacy (or non-HT) long training field (L-LTF), and alegacy (or non-HT) signal (L-SIG) field, and a data field.

The L-STF may include a short training orthogonal frequency divisionmultiplexing symbol (OFDM). The L-STF may be used for frame timingacquisition, automatic gain control (AGC), diversity detection, andcoarse frequency/time synchronization.

The L-LTF may include a long training OFDM symbol. The L-LTF may be usedfor fine frequency/time synchronization and channel estimation.

The L-SIG field may be used to send control information for thedemodulation and decoding of the data field.

The L-SIG field may include a rate field of four bits, a reserved fieldof 1 bit, a length field of 12 bits, a parity bit of 1 bit, and a signaltail field of 6 bits.

The rate field includes transfer rate information, and the length fieldindicates the number of octets of a PSDU.

FIG. 3(b) illustrates an HT mixed format PPDU for supporting both anIEEE 802.11n system and IEEE 802.11a/g system.

Referring to FIG. 3(b), the HT mixed format PPDU is configured toinclude a legacy format preamble including an L-STF, an L-LTF, and anL-SIG field, an HT format preamble including an HT-signal (HT-SIG)field, a HT short training field (HT-STF), and a HT long training field(HT-LTF), and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and are the same as those of the non-HT formatfrom the L-STF to the L-SIG field. An L-STA may interpret a data fieldthrough an L-LTF, an L-LTF, and an L-SIG field although it receives anHT mixed PPDU. In this case, the L-LTF may further include informationfor channel estimation to be performed by an HT-STA in order to receivethe HT mixed PPDU and to demodulate the L-SIG field and the HT-SIGfield.

An HT-STA may be aware of an HT mixed format PPDU using the HT-SIG fieldsubsequent to the legacy fields, and may decode the data field based onthe HT mixed format PPDU.

The HT-LTF may be used for channel estimation for the demodulation ofthe data field. IEEE 802.11n supports single user multi-input andmulti-output (SU-MIMO) and thus may include a plurality of HT-LTFs forchannel estimation with respect to each of data fields transmitted in aplurality of spatial streams.

The HT-LTF may include a data HT-LTF used for channel estimation for aspatial stream and an extension HT-LTF additionally used for fullchannel sounding. Accordingly, a plurality of HT-LTFs may be the same asor greater than the number of transmitted spatial streams.

In the HT mixed format PPDU, the L-STF, the L-LTF, and the L-SIG fieldsare first transmitted so that an L-STA can receive the L-STF, the L-LTF,and the L-SIG fields and obtain data. Thereafter, the HT-SIG field istransmitted for the demodulation and decoding of data transmitted for anHT-STA.

An L-STF, an L-LTF, L-SIG, and HT-SIG fields are transmitted withoutperforming beamforming up to an HT-SIG field so that an L-STA and anHT-STA can receive a corresponding PPDU and obtain data. In an HT-STF,an HT-LTF, and a data field that are subsequently transmitted, radiosignals are transmitted through precoding. In this case, an HT-STF istransmitted so that an STA receiving a corresponding PPDU by performingprecoding may take into considerate a portion whose power is varied byprecoding, and a plurality of HT-LTFs and a data field are subsequentlytransmitted.

FIG. 3(c) illustrates an HT-green field format PPDU (HT-GF format PPDU)for supporting only an IEEE 802.11n system.

Referring to FIG. 3(c), the HT-GF format PPDU includes an HT-GF-STF, anHT-LTF1, an HT-SIG field, a plurality of HT-LTF2s, and a data field.

The HT-GF-STF is used for frame timing acquisition and AGC.

The HT-LTF1 is used for channel estimation.

The HT-SIG field is used for the demodulation and decoding of the datafield.

The HT-LTF2 is used for channel estimation for the demodulation of thedata field. Likewise, an HT-STA uses SU-MIMO. Accordingly, a pluralityof the HT-LTF2s may be configured because channel estimation isnecessary for each of data fields transmitted in a plurality of spatialstreams.

The plurality of HT-LTF2s may include a plurality of data HT-LTFs and aplurality of extension HT-LTFs like the HT-LTF of the HT mixed PPDU.

In FIGS. 3(a) to 3(c), the data field is a payload and may include aservice field, a scrambled PSDU (PSDU) field, tail bits, and paddingbits. All of the bits of the data field are scrambled.

FIG. 3(d) illustrates a service field included in the data field. Theservice field has 20 bits. The 16 bits are assigned No. 0 to No. 15 andare sequentially transmitted from the No. 0 bit. The No. 0 bit to theNo. 6 bit are set to 0 and are used to synchronize a descrambler withina reception stage.

An IEEE 802.11ac WLAN system supports the transmission of a DLmulti-user multiple input multiple output (MU-MIMO) method in which aplurality of STAs accesses a channel at the same time in order toefficiently use a radio channel. In accordance with the MU-MIMOtransmission method, an AP may simultaneously transmit a packet to oneor more STAs that have been subjected to MIMO pairing.

Downlink multi-user transmission (DL MU transmission) means a technologyin which an AP transmits a PPDU to a plurality of non-AP STAs throughthe same time resources using one or more antennas.

Hereinafter, an MU PPDU means a PPDU which delivers one or more PSDUsfor one or more STAs using the MU-MIMO technology or the OFDMAtechnology. Furthermore, an SU PPDU means a PPDU having a format inwhich only one PSDU can be delivered or which does not have a PSDU.

For MU-MIMO transmission, the size of control information transmitted toan STA may be relatively larger than the size of 802.11n controlinformation. Control information additionally required to supportMU-MIMO may include information indicating the number of spatial streamsreceived by each STA and information related to the modulation andcoding of data transmitted to each STA may correspond to the controlinformation, for example.

Accordingly, when MU-MIMO transmission is performed to provide aplurality of STAs with a data service at the same time, the size oftransmitted control information may be increased according to the numberof STAs which receive the control information.

In order to efficiently transmit the control information whose size isincreased as described above, a plurality of pieces of controlinformation required for MU-MIMO transmission may be divided into twotypes of control information: common control information that isrequired for all of STAs in common and dedicated control informationindividually required for a specific STA, and may be transmitted.

FIG. 4 illustrates a VHT format PPDU in a wireless communication systemto which an embodiment of the present invention may be applied.

FIG. 4(a) illustrates a VHT format PPDU for supporting an IEEE 802.11acsystem.

Referring to FIG. 4(a), the VHT format PPDU is configured to include alegacy format preamble including an L-STF, an L-LTF, and an L-SIG field,a VHT format preamble including a VHT-signal-A (VHT-SIG-A) field, a VHTshort training field (VHT-STF), a VHT long training field (VHT-LTF), anda VHT-signal-B (VHT-SIG-B) field, and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and have the same formats as those of the non-HTformat. In this case, the L-LTF may further include information forchannel estimation which will be performed in order to demodulate theL-SIG field and the VHT-SIG-A field.

The L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-A field may berepeated in a 20 MHz channel unit and transmitted. For example, when aPPDU is transmitted through four 20 MHz channels (i.e., an 80 MHzbandwidth), the L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-Afield may be repeated every 20 MHz channel and transmitted.

A VHT-STA may be aware of the VHT format PPDU using the VHT-SIG-A fieldsubsequent to the legacy fields, and may decode the data field based onthe VHT-SIG-A field.

In the VHT format PPDU, the L-STF, the L-LTF, and the L-SIG field arefirst transmitted so that even an L-STA can receive the VHT format PPDUand obtain data. Thereafter, the VHT-SIG-A field is transmitted for thedemodulation and decoding of data transmitted for a VHT-STA.

The VHT-SIG-A field is a field for the transmission of controlinformation that is common to a VHT STAs that are MIMO-paired with anAP, and includes control information for interpreting the received VHTformat PPDU.

The VHT-SIG-A field may include a VHT-SIG-A1 field and a VHT-SIG-A2field.

The VHT-SIG-A1 field may include information about a channel bandwidth(BW) used, information about whether space time block coding (STBC) isapplied or not, a group identifier (ID) for indicating a group ofgrouped STAs in MU-MIMO, information about the number of streams used(the number of space-time streams (NSTS)/part association identifier(AID), and transmit power save forbidden information. In this case, thegroup ID means an identifier assigned to a target transmission STA groupin order to support MU-MIMO transmission, and may indicate whether thepresent MIMO transmission method is MU-MIMO or SU-MIMO.

The VHT-SIG-A2 field may include information about whether a short guardinterval (GI) is used or not, forward error correction (FEC)information, information about a modulation and coding scheme (MCS) fora single user, information about the type of channel coding for multipleusers, beamforming-related information, redundancy bits for cyclicredundancy checking (CRC), the tail bits of a convolutional decoder andso on.

The VHT-STF is used to improve AGC estimation performance in MIMOtransmission.

The VHT-LTF is used for a VHT-STA to estimate an MIMO channel. Since aVHT WLAN system supports MU-MIMO, the VHT-LTF may be configured by thenumber of spatial streams through which a PPDU is transmitted.Additionally, if full channel sounding is supported, the number ofVHT-LTFs may be increased.

The VHT-SIG-B field includes dedicated control information which isnecessary for a plurality of MU-MIMO-paired VHT-STAs to receive a PPDUand to obtain data. Accordingly, only when common control informationincluded in the VHT-SIG-A field indicates that a received PPDU is forMU-MIMO transmission, a VHT-STA may be designed to decode the VHT-SIG-Bfield. In contrast, if common control information indicates that areceived PPDU is for a single VHT-STA (including SU-MIMO), an STA may bedesigned to not decode the VHT-SIG-B field.

The VHT-SIG-B field includes a VHT-SIG-B length field, a VHT-MCS field,a reserved field, and a tail field.

The VHT-SIG-B length field indicates the length of an A-MPDU (prior toend-of-frame (EOF) padding). The VHT-MCS field includes informationabout the modulation, encoding, and rate-matching of each VHT-STA.

The size of the VHT-SIG-B field may be different depending on the type(MU-MIMO or SU-MIMO) of MIMO transmission and a channel bandwidth usedfor PPDU transmission.

FIG. 4(b) illustrates a VHT-SIG-B field according to a PPDU transmissionbandwidth.

Referring to FIG. 4(b), in 40 MHz transmission, VHT-SIG-B bits arerepeated twice. In 80 MHz transmission, VHT-SIG-B bits are repeated fourtimes, and padding bits set to 0 are attached.

In 160 MHz transmission and 80+80 MHz transmission, first, VHT-SIG-Bbits are repeated four times as in the 80 MHz transmission, and paddingbits set to 0 are attached. Furthermore, a total of the 117 bits isrepeated again.

In a system supporting MU-MIMO, in order to transmit PPDUs having thesame size to STAs paired with an AP, information indicating the size ofthe bits of a data field forming the PPDU and/or information indicatingthe size of bit streams forming a specific field may be included in theVHT-SIG-A field.

In this case, an L-SIG field may be used to effectively use a PPDUformat. A length field and a rate field which are included in the L-SIGfield and transmitted so that PPDUs having the same size are transmittedto all of STAs may be used to provide required information. In thiscase, additional padding may be required in the physical layer becausean MAC protocol data unit (MPDU) and/or an aggregate MAC PDU (A-MPDU)are set based on the bytes (or octets) of the MAC layer.

In FIG. 4, the data field is a payload and may include a service field,a scrambled PSDU, tail bits, and padding bits.

An STA needs to determine the format of a received PPDU because severalformats of PPDUs are mixed and used as described above.

In this case, the meaning that a PPDU (or a PPDU format) is determinedmay be various. For example, the meaning that a PPDU is determined mayinclude determining whether a received PPDU is a PPDU capable of beingdecoded (or interpreted) by an STA. Furthermore, the meaning that a PPDUis determined may include determining whether a received PPDU is a PPDUcapable of being supported by an STA. Furthermore, the meaning that aPPDU is determined may include determining that information transmittedthrough a received PPDU is which information.

MAC Frame Format

FIG. 5 illustrates the format of an MAC frame for an IEEE 802.11 systemto which an embodiment of the present invention may be applied.

Referring to FIG. 5, the MAC frame (i.e., an MPDU) includes an MACheader, a frame body, and a frame check sequence (FCS).

The MAC Header is defined as an area, including a frame control field, aduration/ID field, an address 1 field, an address 2 field, an address 3field, a sequence control field, an address 4 field, a QoS controlfield, and an HT control field.

The frame control field includes information about the characteristicsof a corresponding MAC frame.

The duration/ID field may be implemented to have a different valuedepending on the type and subtype of a corresponding MAC frame.

If the type and subtype of a corresponding MAC frame is a PS-poll framefor a power save (PS) operation, the duration/ID field may be configuredto include the association identifier (AID) of an STA that hastransmitted the frame. In the remaining cases, the duration/ID field maybe configured to have a specific duration value depending on the typeand subtype of a corresponding MAC frame. Furthermore, if a frame is anMPDU included in an aggregate-MPDU (A-MPDU) format, the duration/IDfield included in an MAC header may be configured to have the samevalue.

The address 1 field to the address 4 field are used to indicate a BSSID,a source address (SA), a destination address (DA), a transmittingaddress (TA) indicating the address of a transmitting STA, and areceiving address (RA) indicating the address of a receiving STA.

An address field implemented as a TA field may be set as a bandwidthsignaling TA value. In this case, the TA field may indicate that acorresponding MAC frame includes additional information in a scramblingsequence. The bandwidth signaling TA may be represented as the MACaddress of an STA that sends a corresponding MAC frame, butindividual/group bits included in the MAC address may be set as aspecific value (e.g., “1”).

The sequence control field is configured to include a sequence numberand a fragment number. The sequence number may indicate a sequencenumber assigned to a corresponding MAC frame. The fragment number mayindicate the number of each fragment of a corresponding MAC frame.

The QoS control field includes information related to QoS. The QoScontrol field may be included if it indicates a QoS data frame in asubtype subfield.

The HT control field includes control information related to an HTand/or VHT transmission/reception scheme. The HT control field isincluded in a control wrapper frame. Furthermore, the HT control fieldis present in a QoS data frame having an order subfield value of 1 and amanagement frame.

The frame body is defined as an MAC payload. Data to be transmitted in ahigher layer is placed in the frame body. The frame body has a varyingsize. For example, a maximum size of an MPDU may be 11454 octets, and amaximum size of a PPDU may be 5.484 ms.

The FCS is defined as a MAC footer and used for the error search of anMAC frame.

The first three fields (i.e., the frame control field, the duration/IDfield, and Address 1 field) and the last field (i.e., the FCS field)form a minimum frame format and are present in all of frames. Theremaining fields may be present only in a specific frame type.

FIG. 6 is a diagram illustrating a frame control field within an MACframe in a wireless communication system to which an embodiment of thepresent invention may be applied.

Referring to FIG. 6, the frame control field includes a Protocol Versionsubfield, a Type subfield, a Subtype subfield, a To DS subfield, a FromDS subfield, a More Fragments subfield, a Retry subfield, a PowerManagement subfield, a More Data subfield, a Protected Frame subfield,and an Order subfield.

The Protocol Version subfield may indicate the version of a WLANprotocol applied to a corresponding MAC frame.

The Type subfield and the Subtype subfield may be set to indicateinformation that identifies the function of a corresponding MAC frame.

The type of MAC frame may include the three types of management frames,control frames, and data frames.

Furthermore, each of the frame types may be divided into subtypes.

For example, the control frames may include request to send (RTS) frame,a clear-to-send (CTS) frame, an acknowledgment (ACK) frame, a PS-pollframe, a contention free (CF)-end frame, a CF-End+CF-ACK frame, a blockACK request (BAR) frame, a block ACK (BA) frame, a control wrapper(Control+HTcontrol)) frame, a VHT null data packet announcement (NDPA),and a beamforming report poll frame.

The management frames may include a beacon frame, an announcementtraffic indication message (ATIM) frame, a disassociation frame, anassociation request/response frame, a reassociation request/responseframe, a probe request/response frame, an authentication frame, adeauthentication frame, an action frame, an action no ACK frame, and atiming advertisement frame.

The To DS subfield and the From DS subfield may include information thatis necessary to analyze an Address 1 field to an Address 4 fieldincluded in a corresponding MAC frame header. In the case of the controlframe, both the To DS subfield and the From DS subfield are set to “0.”In the case of the management frame, the To DS subfield and the From DSsubfield may be sequentially set to “1” and “0” if a corresponding frameis a QoS management frame (QMF) and may be sequentially set to “0” and“0” if a corresponding frame is not a QMF.

The More Fragments subfield may indicate whether a fragment to betransmitted after a corresponding MAC frame is present or not. The MoreFragments subfield may be set to “1” if another fragment of a currentMSDU or MMPDU is present and may be set to “0” if another fragment of acurrent MSDU or MMPDU is not present.

The Retry subfield may indicate whether the transmission of acorresponding MAC frame is based on the retransmission of a previous MACframe. The Retry subfield may be set to “1” if the transmission of acorresponding MAC frame is based on the retransmission of a previous MACframe and may be set to “0” if the transmission of a corresponding MACframe is not based on the retransmission of a previous MAC frame.

The Power Management subfield may indicate power management mode of anSTA. The Power Management subfield may indicate that an STA switches topower saving mode if the Power Management subfield value is “1.”

The More Data subfield may indicate whether an MAC frame to beadditionally transmitted is present or not. The More Data subfield maybe set to “1” if an MAC frame to be additionally transmitted is presentand may be set to “0” if an MAC frame to be additionally transmitted isnot present.

The Protected Frame subfield may indicate whether a Frame Body field hasbeen encrypted. The Protected Frame subfield may be set to “1” if theFrame Body field includes information processed by a cryptographicencapsulation algorithm and may be set to “0” if the Frame Body fielddoes not include information processed by a cryptographic encapsulationalgorithm.

The pieces of information included in each of the aforementioned fieldsmay comply with the definition of the IEEE 802.11 system. Furthermore,the aforementioned fields correspond to an example of fields which maybe included in an MAC frame, but the present invention is not limitedthereto. That is, each of the aforementioned fields may be replaced withanother field or an additional field may be further included and all ofthe fields may not be essentially included.

FIG. 7 illustrates the VHT format of an HT control field in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

Referring to FIG. 7, the HT control field may include a VHT subfield, anHT control middle subfield, an AC constraint subfield, and a reversedirection grant (RDG)/more PPDU subfield.

The VHT subfield indicates whether the HT control field has the formatof an HT control field for VHT (VHT=1) or has the format of an HTcontrol field for HT (VHT=0). In FIG. 9, it is assumed that the HTcontrol field is an HT control field for VHT (i.e., VHT=1). The HTcontrol field for VHT may be called a VHT control field.

The HT control middle subfield may be implemented to a different formatdepending on the indication of a VHT subfield. The HT control middlesubfield is described in detail later.

The AC constraint subfield indicates whether the mapped access category(AC) of a reverse direction (RD) data frame is constrained to a singleAC.

The RDG/more PPDU subfield may be differently interpreted depending onwhether a corresponding field is transmitted by an RD initiator or an RDresponder.

Assuming that a corresponding field is transmitted by an RD initiator,the RDG/more PPDU subfield is set as “1” if an RDG is present, and theRDG/more PPDU subfield is set as “0” if an RDG is not present. Assumingthat a corresponding field is transmitted by an RD responder, theRDG/more PPDU subfield is set as “1” if a PPDU including thecorresponding subfield is the last frame transmitted by the RDresponder, and the RDG/more PPDU subfield is set as “0” if another PPDUis transmitted. As described above, the HT control middle subfield maybe implemented to a different format depending on the indication of aVHT subfield.

The HT control middle subfield of an HT control field for VHT mayinclude a reserved bit subfield, a modulation and coding scheme (MCS)feedback request (MRQ) subfield, an MRQ sequence identifier(MSI)/space-time block coding (STBC) subfield, an MCS feedback sequenceidentifier (MFSI)/least significant bit (LSB) of group ID (GID-L)subfield, an MCS feedback (MFB) subfield, a most significant Bit (MSB)of group ID (GID-H) subfield, a coding type subfield, a feedbacktransmission type (FB Tx type) subfield, and an unsolicited MFBsubfield.

Furthermore, the MFB subfield may include the number of VHT space timestreams (NUM_STS) subfield, a VHT-MCS subfield, a bandwidth (BW)subfield, and a signal to noise ratio (SNR) subfield.

The NUM_STS subfield indicates the number of recommended spatialstreams. The VHT-MCS subfield indicates a recommended MCS. The BWsubfield indicates bandwidth information related to a recommended MCS.The SNR subfield indicates an average SNR value of data subcarriers andspatial streams.

The information included in each of the aforementioned fields may complywith the definition of an IEEE 802.11 system. Furthermore, each of theaforementioned fields corresponds to an example of fields which may beincluded in an MAC frame and is not limited thereto. That is, each ofthe aforementioned fields may be substituted with another field,additional fields may be further included, and all of the fields may notbe essentially included.

Medium Access Mechanism

In IEEE 802.11, communication is basically different from that of awired channel environment because it is performed in a shared wirelessmedium.

In a wired channel environment, communication is possible based oncarrier sense multiple access/collision detection (CSMA/CD). Forexample, when a signal is once transmitted by a transmission stage, itis transmitted up to a reception stage without experiencing great signalattenuation because there is no great change in a channel environment.In this case, when a collision between two or more signals is detected,detection is possible. The reason for this is that power detected by thereception stage becomes instantly higher than power transmitted by thetransmission stage. In a radio channel environment, however, sincevarious factors (e.g., signal attenuation is great depending on thedistance or instant deep fading may be generated) affect a channel, atransmission stage is unable to accurately perform carrier sensingregarding whether a signal has been correctly transmitted by a receptionstage or a collision has been generated.

Accordingly, in a WLAN system according to IEEE 802.11, a carrier sensemultiple access with collision avoidance (CSMA/CA) mechanism has beenintroduced as the basic access mechanism of MAC. The CAMA/CA mechanismis also called a distributed coordination function (DCF) of IEEE 802.11MAC, and basically adopts a “listen before talk” access mechanism. Inaccordance with such a type of access mechanism, an AP and/or an STAperform clear channel assessment (CCA) for sensing a radio channel or amedium for a specific time interval (e.g., a DCF inter-frame space(DIFS)) prior to transmission. If, as a result of the sensing, themedium is determined to be an idle state, the AP and/or the STA startsto transmit a frame through the corresponding medium. In contrast, if,as a result of the sensing, the medium is determined to be a busy state(or an occupied status), the AP and/or the STA do not start theirtransmission, may wait for a delay time (e.g., a random backoff period)for medium access in addition to the DIFS assuming that several STAsalready wait for in order to use the corresponding medium, and may thenattempt frame transmission.

Assuming that several STAs trying to transmit frames are present, theywill wait for different times because the STAs stochastically havedifferent backoff period values and will attempt frame transmission. Inthis case, a collision can be minimized by applying the random backoffperiod.

Furthermore, the IEEE 802.11 MAC protocol provides a hybrid coordinationfunction (HCF). The HCF is based on a DCF and a point coordinationfunction (PCF). The PCF is a polling-based synchronous access method,and refers to a method for periodically performing polling so that allof receiving APs and/or STAs can receive a data frame. Furthermore, theHCF has enhanced distributed channel access (EDCA) and HCF controlledchannel access (HCCA). In EDCA, a provider performs an access method forproviding a data frame to multiple users on a contention basis. In HCCA,a non-contention-based channel access method using a polling mechanismis used. Furthermore, the HCF includes a medium access mechanism forimproving the quality of service (QoS) of a WLAN, and may transmit QoSdata in both a contention period (CP) and a contention-free period(CFP).

FIG. 8 is a diagram illustrating a random backoff period and a frametransmission procedure in a wireless communication system to which anembodiment of the present invention may be applied.

When a specific medium switches from an occupied (or busy) state to anidle state, several STAs may attempt to transmit data (or frames). Inthis case, as a scheme for minimizing a collision, each of the STAs mayselect a random backoff count, may wait for a slot time corresponding tothe selected random backoff count, and may attempt transmission. Therandom backoff count has a pseudo-random integer value and may bedetermined as one of uniformly distributed values in 0 to a contentionwindow (CW) range. In this case, the CW is a CW parameter value. In theCW parameter, CW_min is given as an initial value. If transmission fails(e.g., if ACK for a transmitted frame is not received), the CW_min mayhave a twice value. If the CW parameter becomes CW_max, it may maintainthe CW_max value until data transmission is successful, and the datatransmission may be attempted. If the data transmission is successful,the CW parameter is reset to a CW_min value. The CW, CW_min, and CW_maxvalues may be set to 2^n−1 (n=0, 1, 2, . . . ,).

When a random backoff process starts, an STA counts down a backoff slotbased on a determined backoff count value and continues to monitor amedium during the countdown. When the medium is monitored as a busystate, the STA stops the countdown and waits. When the medium becomes anidle state, the STA resumes the countdown.

In the example of FIG. 8, when a packet to be transmitted in the MAC ofan STA 3 is reached, the STA 3 may check that a medium is an idle stateby a DIFS and may immediately transmit a frame.

The remaining STAs monitor that the medium is the busy state and wait.In the meantime, data to be transmitted by each of an STA 1, an STA 2,and an STA 5 may be generated. When the medium is monitored as an idlestate, each of the STAs waits for a DIFS and counts down a backoff slotbased on each selected random backoff count value.

The example of FIG. 8 shows that the STA 2 has selected the smallestbackoff count value and the STA 1 has selected the greatest backoffcount value. That is, FIG. 8 illustrates that the remaining backoff timeof the STA 5 is shorter than the remaining backoff time of the STA 1 ata point of time at which the STA 2 finishes a backoff count and startsframe transmission.

The STA 1 and the STA 5 stop countdown and wait while the STA 2 occupiesthe medium. When the occupation of the medium by the STA is finished andthe medium becomes an idle state again, each of the STA 1 and the STA 5waits for a DIFS and resumes the stopped backoff count. That is, each ofthe STA 1 and the STA 5 may start frame transmission after counting downthe remaining backoff slot corresponding to the remaining backoff time.The STA 5 starts frame transmission because the STA 5 has a shorterremaining backoff time than the STA 1.

While the STA 2 occupies the medium, data to be transmitted by an STA 4may be generated. In this case, from a standpoint of the STA 4, when themedium becomes an idle state, the STA 4 waits for a DIFS and counts downa backoff slot corresponding to its selected random backoff count value.

FIG. 8 shows an example in which the remaining backoff time of the STA 5coincides with the random backoff count value of the STA 4. In thiscase, a collision may be generated between the STA 4 and the STA 5. Whena collision is generated, both the STA 4 and the STA 5 do not receiveACK, so data transmission fails. In this case, each of the STA 4 and theSTA 5 doubles its CW value, select a random backoff count value, andcounts down a backoff slot.

The STA 1 waits while the medium is the busy state due to thetransmission of the STA 4 and the STA 5. When the medium becomes an idlestate, the STA 1 may wait for a DIFS and start frame transmission afterthe remaining backoff time elapses.

The CSMA/CA mechanism includes virtual carrier sensing in addition tophysical carrier sensing in which an AP and/or an STA directly sense amedium.

Virtual carrier sensing is for supplementing a problem which may begenerated in terms of medium access, such as a hidden node problem. Forthe virtual carrier sensing, the MAC of a WLAN system uses a networkallocation vector (NAV). The NAV is a value indicated by an AP and/or anSTA which now uses a medium or has the right to use the medium in orderto notify another AP and/or STA of the remaining time until the mediumbecomes an available state. Accordingly, a value set as the NAVcorresponds to the period in which a medium is reserved to be used by anAP and/or an STA that transmit corresponding frames. An STA thatreceives an NAV value is prohibited from accessing the medium during thecorresponding period. The NAV may be set based on the value of theduration field of the MAC header of a frame, for example.

An APand/or an STA may perform a procedure for exchanging a request tosend (RTS) frame and a clear to send (CTS) frame in order to providenotification that they will access a medium. The RTS frame and the CTSframe include information indicating a temporal section in which awireless medium required to transmit/receive an ACK frame has beenreserved to be accessed if substantial data frame transmission and anacknowledgement response (ACK) are supported. Another STA which hasreceived an RTS frame from an AP and/or an STA attempting to send aframe or which has received a CTS frame transmitted by an STA to which aframe will be transmitted may be configured to not access a mediumduring a temporal section indicated by information included in theRTS/CTS frame. This may be implemented by setting the NAV during a timeinterval.

Interframe Space (IFS)

A time interval between frames is defined as an interframe space (IFS).An STA may determine whether a channel is used during an IFS timeinterval through carrier sensing (including physical carrier and virtualcarrier sensing). In an 802.11 WLAN system, a plurality of IFSs isdefined in order to provide a priority level by which a wireless mediumis occupied.

FIG. 9 is a diagram illustrating an IFS relation in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

All of pieces of timing may be determined with reference to physicallayer interface primitives, that is, a PHY-TXEND.confirm primitive, aPHYTXSTART.confirm primitive, a PHY-RXSTART.indication primitive, and aPHY-RXEND.indication primitive.

An interframe space (IFS) depending on an IFS type is as follows.

a) A reduced interframe space (IFS) (RIFS)

b) A short interframe space (IFS) (SIFS)

c) A PCF interframe space (IFS) (PIFS)

d) A DCF interframe space (IFS) (DIFS)

e) An arbitration interframe space (IFS) (AIFS)

f) An extended interframe space (IFS) (EIFS)

Different IFSs are determined based on attributes specified by aphysical layer regardless of the bit rate of an STA. IFS timing isdefined as a time gap on a medium. IFS timing other than an AIFS isfixed for each physical layer.

The SIFS is used to transmits a PPDU including an ACK frame, a CTSframe, a block ACK request (BlockAckReq) frame, or a block ACK(BlockAck) frame, that is, an instant response to an A-MPDU, the secondor consecutive MPDU of a fragment burst, and a response from an STA withrespect to polling according to a PCF. The SIFS has the highestpriority. Furthermore, the SIFS may be used for the point coordinator offrames regardless of the type of frame during a non-contention period(CFP) time. The SIFS indicates the time prior to the start of the firstsymbol of the preamble of a next frame which is subsequent to the end ofthe last symbol of a previous frame or from signal extension (ifpresent).

SIFS timing is achieved when the transmission of consecutive frames isstarted in a Tx SIFS slot boundary.

The SIFS is the shortest in IFS between transmissions from differentSTAs. The SIFS may be used if an STA occupying a medium needs tomaintain the occupation of the medium during the period in which theframe exchange sequence is performed.

Other STAs required to wait so that a medium becomes an idle state for alonger gap can be prevented from attempting to use the medium becausethe smallest gap between transmissions within a frame exchange sequenceis used. Accordingly, priority may be assigned in completing a frameexchange sequence that is in progress.

The PIFS is used to obtain priority in accessing a medium.

The PIFS may be used in the following cases.

-   -   An STA operating under a PCF    -   An STA sending a channel switch announcement frame    -   An STA sending a traffic indication map (TIM) frame    -   A hybrid coordinator (HC) starting a CFP or transmission        opportunity (TXOP)    -   An HC or non-AP QoS STA, that is, a TXOP holder polled for        recovering from the absence of expected reception within a        controlled access phase (CAP)    -   An HT STA using dual CTS protection before sending CTS2    -   A TXOP holder for continuous transmission after a transmission        failure    -   A reverse direction (RD) initiator for continuous transmission        using error recovery    -   An HT AP during a PSMP sequence in which a power save multi-poll        (PSMP) recovery frame is transmitted    -   An HT AT performing CCA within a secondary channel before        sending a 40 MHz mask PPDU using EDCA channel access

In the illustrated examples, an STA using the PIFS starts transmissionafter a carrier sense (CS) mechanism for determining that a medium is anidle state in a Tx PIFS slot boundary other than the case where CCA isperformed in a secondary channel.

The DIFS may be used by an STA which operates to send a data frame(MPDU) and a MAC management protocol data unit management (MMPDU) frameunder the DCF. An STA using the DCF may transmit data in a TxDIFS slotboundary if a medium is determined to be an idle state through a carriersense (CS) mechanism after an accurately received frame and a backofftime expire. In this case, the accurately received frame means a frameindicating that the PHY-RXEND.indication primitive does not indicate anerror and an FCS indicates that the frame is not an error (i.e., errorfree).

An SIFS time (“aSIFSTime”) and a slot time (“aSlotTime”) may bedetermined for each physical layer. The SIFS time has a fixed value, butthe slot time may be dynamically changed depending on a change in thewireless delay time “aAirPropagationTime.”

Block ACK Procedure

FIG. 10 is a diagram illustrating a downlink MU-MIMO transmissionprocess in a wireless communication system to which an embodiment of thepresent invention may be applied.

In 802.11ac, MU-MIMO is defined in downlink from an AP to a client(i.e., a non-AP STA). In this case, a multi-user frame is transmitted tomultiple recipients at the same time, but acknowledgement (ACK) needs tobe individually transmitted in uplink.

All of MPDUs transmitted within a VHT MU PPDU based on 802.11ac areincluded in an A-MPDU. Accordingly, a response to the A-MPDU within theVHT MU PPDU other than an immediate response to the VHT MU PPDU istransmitted in response to a block ACK request (BAR) frame by the AP.

First, the AP sends the VHT MU PPDU (i.e., a preamble and data) to allof recipients (i.e., an STA 1, an STA 2, and an STA 3). The VHT MU PPDUincludes a VHT A-MPDU transmitted to each STA.

The STA 1 that has received the VHT MU PPDU from the AP sends a blockacknowledgement (BA) frame to the AP after an SIFS. The BA frame isdescribed later in detail.

The AP that has received the BA from the STA 1 sends a blockacknowledgement request (BAR) frame to the STA 2 after an SIFS. The STA2 sends a BA frame to the AP after an SIFS. The AP that has received theBA frame from the STA 2 sends a BAR frame to the STA 3 after an SIFS.The STA 3 sends a BA frame to the AP after an SIFS.

When such a process is performed by all of the STAs, the AP sends a nextMU PPDU to all of the STAs.

High Efficiency (HE, 802.11ax) System

A next-generation WLAN system is described below. The next-generationWLAN system is a next-generation WIFI system. By way of example, IEEE802.11ax may be described as an embodiment of the next-generation WIFIsystem. In this specification, the following next-generation WLAN systemis named a high efficiency (HE) system, and the frame, PPDU, etc. of thesystem may be referred to as an HE frame, HE PPDU, HE preamble, HE-SIGfield, HE-STF, HE-LTF and so on.

The aforementioned description of an existing WLAN system, such as anVHT system, may be applied to contents that have not been additionallydescribed with respect to the HE system. For example, the aforementioneddescription of the VHT-SIG A field, the VHT-STF, the VHT-LTF, and theVHT-SIG-B field may be applied to the HE-SIG A field, the HE-STF, theHE-LTF, and the HE-SIG-B field. The HE frame, preamble, etc. of theproposed HE system may also be used in other wireless communication orcellular systems. As described above, an HE STA may be a non-AP STA orAP STA. In this specification, although an STA is described, such an STAdevice may also denote an HE STA device.

An HE format PPDU for HEW may basically include a legacy part (L-part),an HE-part, and a data field (HE-data).

The L-part includes an L-STF field, an L-LTF field, and an L-SIG fieldlike a form maintained in an existing WLAN system. The L-STF field, theL-LTF field, and the L-SIG field may also be called a legacy preamble.

The HE-part is a part newly defined for the 802.11ax standard, and mayinclude an HE-STF field, an HE-SIG field, and an HE-LTF field. TheHE-SIG field in addition to the HE-STF field and the HE-LTF field mayalso be called an HE-preamble.

Furthermore, the legacy preamble and the HE preamble may be collectivelycalled a physical (PHY) preamble/physical preamble.

The HE-SIG field may include information (e.g., OFDMA, UL MU MIMO, andan improved MCS) for decoding the HE-data field.

The L-part and the HE-part may have different fast Fourier transform(FFT) sizes (i.e., subcarrier spacing) and may use different cyclicprefixes.

A (4×) FFT size four times greater than that of a legacy WLAN system maybe used in the 802.11ax system. That is, the L-part may have a 1× symbolstructure, and the HE-part (more specifically, the HE-preamble and theHE-data) may have a 4×symbol structure. In this case, FFT of a 1×, 2× or4×size indicates a relative size for a legacy WLAN system (e.g., IEEE802.11a, 802.11n, and 802.11ac).

For example, if FFT sizes used in the L-part are 64, 128, 256, and 512in 20 MHz, 40 MHz, 80 MHz, and 200 MHz, respectively, FFT sizes used inthe HE-part may be 256, 512, 1024, and 2048 in 20 MHz, 40 MHz, 80 MHz,and 200 MHz, respectively.

As described above, if an FFT size is increased compared to a legacyWLAN system, subcarrier frequency spacing is reduced. Accordingly, thenumber of subcarriers per unit frequency is increased, but the length ofan OFDM symbol is lengthened.

That is, if a larger FFT size is used, it means that subcarrier spacingis narrowed. Likewise, this means that an inverse discrete Fouriertransform (IDFT)/discrete Fourier transform (DFT) period is increased.In this case, the IDFT/DFT period may mean the length of an OFDM symbolother than a guard interval (GI).

Accordingly, if an FFT size four times greater than that of the L-partis used in the HE-part (more specifically, the HE-preamble and theHE-data), the subcarrier spacing of the HE-part becomes ¼ times thesubcarrier spacing of the L-part and the IDFT/DFT period of the HE-partbecomes four times the IDFT/DFT period of the L-part. For example, ifthe subcarrier spacing of the L-part is 312.5 kHz (=20 MHz/64, 40MHz/128, 80 MHz/256 and/or 200 MHz/512), the subcarrier spacing of theHE-part may be 78.125 kHz (=20 MHz/256, 40 MHz/512, 80 MHz/1024 and/or200 MHz/2048). Furthermore, if the IDFT/DFT period of the L-part is 3.2μs (=1/312.5 kHz), the IDFT/DFT period of the HE-part may be 12.8 μs(=1/78.125 kHz).

Furthermore, in the 802.11ax system, a resource unit may be allocatedfor each STA through an OFDMA scheme. In an embodiment, the resourceunit may be allocated to an STA in a 26-tone, 52-tone or 106-tone unitwith respect to a 20 MHz band, in a 26-tone, 52-tone, 106-tone or242-tone unit with respect to a 40 MHz band, and in a 26-tone, 52-tone,106-tone, 242-tone or 484-tone unit with respect to an 80 MHz band.Furthermore, the resource unit may be allocated to an STA in a 26-tone,52-tone, 106-tone, 242-tone, 484-tone or 996-tone unit with respect to a160 MHz band.

In this specification, the HE-SIG-A is an HE-SIG1, and thus the HE-SIG-Bmay be referred to as an HE-SIG2.

FIG. 11 shows an HE PPDU format according to an embodiment of thepresent invention.

In the embodiment of FIG. 11, an HE-SIG1 is placed behind an L-part (anL-STF, an L-LTF, and an L-SIG) using legacy numerology. As in theL-part, the HE-SIG1 may be duplicated in a 20 MHz unit. An HE-SIG-1field may include common information (a BW, a GI length, a BSS index,CRC, tails, etc.). 4×FFT is applied to HE-DATA, and thus 1024 FFT may beused.

FIG. 12 shows an HE-PPDU format according to another embodiment of thepresent invention.

In the embodiment of FIG. 12, an HE-SIG1 may further include userallocation information (e.g., the ID (e.g., PAID or GID) and resourceallocation information of an STA and N_sts) in addition to commoninformation. Furthermore, an HE-SIG-A may be transmitted according tothe resource allocation of OFDMA. In the case of MU-MIMO, an HE-SIG2 maybe distinguished by an STA through SDM. The HE-SIG2 may includeadditional user allocation information (e.g., an MCS, coding, STBC, andTSBF).

FIG. 13 shows an HE-PPDU format according to another embodiment of thepresent invention.

In the embodiment of FIG. 13, a HE-SIG1 field and an HE-SIG2 field maybe included after a legacy preamble, and an HE-STF and an HE-LTF may besubsequently included. The HE-SIG2 may be transmitted after an HE-SIG1over a full band using the information (numerology) of the HE-SIG1. TheHE-SIG2 may include user allocation information (e.g., the ID (e.g.,PAID or GID) and resource allocation information of an STA and N_sts).

The HE-STF and the HE-LTF may be included in a corresponding resourceunit band according to user-based resource allocation of an OFDMA schemeas in FIG. 12.

The TXOP protection of an UL MU procedure of the 802.11ax system isdescribed below.

An HE system supports UL MU data transmission. More specifically, in ULMU, resources according to each user may be allocated according to anOFDMA scheme and UL MU transmission may be initiated through a triggerframe. For example, an AP STA may send a trigger frame indicative ofresource allocation and scheduling to STAs on which UL MU datatransmission is to be performed. The STAs that have received the triggerframe may send uplink data at the same time.

FIG. 14 shows the UL MU data transmission of an HE system according toan embodiment of the present invention.

In FIG. 14, MU STAs, such as the MU STA 1 and MU STA 2 of a BSS, performUL MU data transmission in response to the trigger frame of an AP. Inthis case, another STA1 within the BSS may not recognize the presence ofan UL MU frame. In this case, another STA1 may send UL data after anEIFS, thereby generating a collision. In llax UL MU transmission, an ULMU frame/packet may have a longer length than an existing ACK frame, andthus the influence of the collision may be more severe.

FIG. 15 shows the UL MU data transmission of an HE system according toan embodiment of the present invention.

In FIG. 15, MU STAs, such as the MU STA 1 and MU STA 2 of a BSS, performUL MU data transmission in response to the trigger frame of an AP. Inthis case, another STA2 of an OBSS may not be aware of the presence of atrigger frame and/or ACK frame. In this case, another STA2 may attemptto transmit its own packet after an EIFS from the end of an UL MU frame.At this time, a collision with a DL MU ACK frame may be generated. Morespecifically, the influence of the collision may be further increaseddepending on the length of the DL MU ACK frame.

As described with reference to FIGS. 14 and 15, transmission opportunity(TXOP) protection for UL MU transmission needs to be performed. TXOPindicates the time interval in which a specific quality of service (QoS)STA has the right to initiate a frame exchange sequence on a radiomedium. For the TXOP protection, a method for performing the TXOPprotection using an L-SIG or sending a TXOP length in the HE-SIG-A fieldof a trigger frame and/or UL/DL MU frame may be used.

FIGS. 16 to 18 show the transmission of information by an STA in receive(Rx) operating mode according to an embodiment of the present invention.

In FIGS. 16 to 18, communication between an AP STA and a non-AP STA hasbeen illustrated, for convenience of a description, but the AP STA maybe a non-AP STA.

An STA may include Rx operating mode information in the MAC header orPSDU of a control frame, data frame or management frame in order tochange Rx operating mode, and may send the control frame, data frame ormanagement frame. In this case, the Rx operating mode information ismode information on which a receiver receives data, and may include atleast one of channel BW information, tone information, RU information,and receiving stream number information. Furthermore, in order toindicate a request for the Rx operating mode change, Rx mode requestinformation/bit may be additionally defined and included. The STA mayindicate whether the Rx operating mode information is included in theMAC header using the reserved bits of an HT variant field or VHT variantfield of the HT control field of the MAC header.

If a specific STA1 sends Rx operating mode information, whether an STA2that has received the Rx operating mode information accepts or deniesthe Rx operating mode information may be indicated using a BA/ACK (orM-BA or OFDMA-BA) frame. The STA2 may send information about theacceptance or denial of the Rx operating mode information using thereserved bits of the BA and/or the ACK frame.

Alternatively, a specific STA2 may set an Rx mode request bit to 1 andtransmit the Rx mode request bit in order to change Rx operating mode ofthe STA1. The STA1 may indicate whether it accepts or denies Rxoperating mode, requested by the STA2, using a BA/ACK (or M-BA orOFDMA-BA) frame. The STA1 may send information about the acceptance ordenial of the reception operating request using the reserved bits of theBA and/or the ACK frame.

In the embodiment of FIG. 16, the STA may send the Rx operating modeinformation while sending a data frame. The data frame may be an UL dataframe or a DL data frame. The Rx operating mode information may indicatethat the receiving number of spatial streams (Rx NSS)=2 and a receiving(Rx) channel bandwidth=40 MHz. An AP may indicate that it accepts the Rxoperating mode information using transmitted BA. Accordingly, when theAP subsequently sends data, the AP may send data in Tx mode of Rx NSS=2and BW=40 MHz.

In the embodiment of FIG. 17, an STA may send Rx operating modeinformation for changing Rx mode into Rx NSS=1 and Rx channel bandwidth=20 MHz along with a data frame. Furthermore, when an AP accepts the Rxmode change, the AP may send such Rx mode change acceptance through BA.Accordingly, the AP sends data with Rx NSS=1 and Rx channel bandwidth=20 MHz. In this case, the AP may send the Rx operating mode informationfor changing Rx mode into Rx NSS=3 and Rx BW=80 MHz along with the data.When the STA accepts the Rx mode change, the STA may send Rx mode changeacceptance along with BA. Furthermore, the AP may send data with Nss=3and BW=80 MHz, that is, changed reception mode.

FIG. 18 shows an embodiment in which stop/outage duration is taken intoconsideration in the embodiments of FIGS. 16 and 17. Furthermore, FIG.18 shows an embodiment in which an AP requests an Rx mode change from anSTA. That is, FIG. 18 shows an embodiment in which an STA sends Rxoperating mode information for receiving data and an STA sends Rxoperating mode information for changing operating mode of a target STAin order to send data.

When an AP sends a data frame, including Rx operating mode information,to an STA, the STA may send ACK/BA and perform an NSS and/or BSS change.Accordingly, the AP may send data after a specific time interval bytaking into consideration time for an Rx mode determination or Rx modechange. Such a time may be referred to as an outage time or outageduration/interval. The outage duration means duration in which a PPDU istransmitted to an STA to which an AP/STA has sent Rx operating modeinformation in changed operating mode after an ACK frame is transmittedor received. As in FIG. 18, time duration in which Rx operating mode ischanged may be referred to as a transition time. During such atransition time, a transmitting STA may stop data transmission.

FIG. 19 shows a multi-STA BA frame format according to an embodiment ofthe present invention.

In the embodiment of FIG. 19, a multi-traffic identifier (TID) BA frameincludes an identifier indicating that the frame is a multi STA BA. BAinformation fields may be addressed to different STAs. The data ofB0-B10 of a Per TID Info field may carry an AID to identify an intendedreceiver of a BA information field. Furthermore, an STA may define thesignaling of a multi-STA BA frame indicative of ACK using the data ofB11. In the embodiment of FIG. 19, when B11 is set, the BlockAck Bitmapand SC subfield of a BA Info field to be described later are notpresent, and the BA Info field may indicate the ACK of a single MPDU orall of MPDUs.

Referring to FIG. 19, a block ACK BA frame includes a Frame Controlfield, a Duration/ID field, a receive address (RA) field, a transmitaddress (TA) field, a BA control field, a BA information field, and aframe check sequence (FCS).

The RA field may be set as the address of an STA that has requestedblock ACK.

The TA field may be set as the address of an STA that sends the BAframe. subfield, a Compressed Bitmap subfield, a Reserved subfield, anda TID information (TID_Info) subfield.

Table 1 illustrates the BA control field. In Table 1, the number of bitsallocated to a reserved subfield has been illustrated as being 9, but 8bits may be allocated depending on an embodiment.

Table 1 illustrates the BA control field. In Table 1, the number of bitsallocated to a reserved subfield has been illustrated as being 9, but 8bits may be allocated depending on an embodiment.

TABLE 1 SUBFIELD BIT DESCRIPTION BA Ack 1 Set to “0” when a senderrequests immediate Policy Ack for data transmission. Set to “1” when asender does not request immediate Ack for data transmission. Multi-TID 1Indicate the type of BA frame depending on values Compressed 1 of theMulti-TID subfield and Compressed Bitmap Bitmap subfield. 00: Basic BA01: Compressed BA 10: Reserved value 11: Multi-TID BA Reserved 9TID_Info 4 The meaning of the TID_Info field is determined depending onthe type of BA frame. Include TID in which the BA frame is transmittedin the case of the Basic BA frame and the Compressed BA frame. Includethe number of TID in the case of the Multi-TID BA frame

In the case of the Multi-TID BA frame, the BA Information field includesa repetition of a Per TID Info subfield, a Block Ack Starting SequenceControl subfield, and a Block Ack Bitmap subfield for one or more TID,and includes an increasing sequence of TIDs.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield includes a TID value.

The Block Ack Starting Sequence Control subfield includes a FragmentNumber and a Starting Sequence Number subfield as described above. TheFragment Number subfield is set to 0. The Starting Sequence Controlsubfield includes the sequence number of the first MSDU or A-MSDU bywhich a corresponding BA frame is transmitted.

The Block Ack Bitmap subfield includes a length of 8 octets. In theBlock Ack Bitmap subfield, a value “1” may indicate that a single MSDUor A-MSDU corresponding to a corresponding bit position has beensuccessfully received. A value “0” may indicate that a single MSDU orA-MSDU corresponding to a corresponding bit position has not beensuccessfully received.

As described above, when an STA performs spatial multiplexing or doesnot perform spatial multiplexing, it may change the number of active Rxchains for power saving depending on the number of Rx special streams(NSS) and an Rx channel bandwidth. For such an Rx mode change, signalsmay be exchanged between an AP and the STA. More specifically, thisspecification proposes a method for transmitting and receivinginformation for a change of the Rx NSS and Rx BW in order to change thenumber of active Rx chains between an AP and an STA. Furthermore, thisspecification also proposes a method for changing Tx operating mode ofan AP preferred by an STA for a Tx operating mode change of the AP. Amethod for changing operating mode, which is proposed according to anembodiment of the present invention, is described below for each item.

1. Method for Indicating Rx Operating Mode

As described above with reference to FIGS. 16 to 18, the STA2 that hasreceived the Rx operating mode information from the STA1 may indicatewhether it will accept or deny the operating mode change using thereserved bit of BA/ACK. An embodiment of the present inventionadditionally proposes a method for indicating another Rx operating modeto be changed by the STA1 if the STA1 denies an operating mode change.

To this end, the reserved bits of the BA control field shown in Table 1may be used as follows. In Table 1, the reserved bits of the BA controlfield have been illustrated as being 9 bits, but may be 8 bits in someembodiments. An example in which the reserved bits of the BA controlfield are 8 bits is described below. In an embodiment, the Rx operatingmode information may be configured as follows. In this case, informationabout such a configuration may be used as Tx operating mode information,which will be described later.

-   -   1 bit: An indicator indicating whether the STA2 accepts or        denies an operating mode change requested by the STA1.    -   1 bit: An indicator indicating whether the STA2 sends proper Rx        operating mode information for an SAT1. This may be applied        similar to the Rx mode request bit.    -   3 bits: The number of received spatial streams (NSS) information    -   3 bits: Rx channel bandwidth information

In an embodiment, when the STA2 accepts an Rx operating mode changerequested by the STA1, an indicator indicating whether proper Rxoperating mode information has been transmitted, NSS information, and Rxchannel bandwidth information may be omitted. Alternatively, when theSTA2 accepts an Rx operating mode change requested by the STA1, it maysend NSS information and channel bandwidth information corresponding toaccepted operating mode. This may be used to confirm the Rx operatingmode information transmitted by the STA1. For example, the pieces ofinformation may be signaled additionally using the reserved bits orreserved values of the Multi-TID field, compressed bitmap field, and GCRfield.

In an embodiment, the STA2 may send at least one of information aboutwhether it accepts operating mode information requested by the STA1 ornot, information about whether it has sent proper operating modeinformation or not, and proper operating mode information—NSSinformation and BW information using the BA information field of FIG.19. For example, the STA2 may send at least one of information aboutwhether it accepts operating mode information requested by the STA1 ornot, information about whether it has sent proper operating modeinformation or not, and proper operating mode information-NSSinformation and BW information using the M-BA frame. The STA2 maypreviously define the TID value of the BA information field as aspecific measured value (e.g., all 0 or all 1). If the TID value is setas such a specific value, the STA2 may send the aforementionedinformation using the Block Ack Starting Sequence Control field and/orthe Block Ack Bitmap field. In other words, the STA2 may indicate thatit sends new Rx operating mode using a previously defined TID value. Inthis case, a field included in the TID Value field instead of the BlockAck Starting Sequence Control field and/or the Block Ack Bitmap fieldmay be newly defined as a field indicative of Rx operating modeinformation.

The STA2 may send new Rx operating mode information for the STA1 usingthe MAC header of a frame (e.g., a data frame) or BA/ACK (or M-BA orOFDMA-BA) frame transmitted by the STA2. Alternatively, the STA2 maysend new operating mode information by piggybacking it on a transmissionframe. In an embodiment, the Rx operating mode information may includechannel information and spatial stream number information for sending,by the STA2, a trigger frame for the STA1 that will perform UL MUtransmission. If a specific STA1 applies UL MU for the STA1, the STA2may send BW or channel information and NSS information on which a nexttrigger frame will be transmitted to the STA1 using a BA/ACK frame, thatis, a response frame for an UL data frame transmitted by the STA1, or aframe transmitted from the STA2 to the STA1. The STA1 may perform asubsequent reception operation according to Rx operating modetransmitted by the STA2.

2. Method for Indicating Tx Operating Mode

An embodiment of the present invention proposes a method for indicatingTx operating mode in addition to the transmission and reception of Rxoperating information for an Rx operating mode change.

In the 802.11ax system, for a coverage extension in outdoorenvironments, an STA may randomly access specific resources in uplink.In this case, Tx operating mode transmitted by the STA may need to bechanged. For example, only a specific STA of STAs that have performedrandom access may access an AP using only 26 tones. In this case, if theSTA receives a trigger frame for the random access and sends an ULframe, it may send its own Tx operating mode information using thereserved bits of a field or frame (e.g., ACK/BA, data or a buffer statusreport) of the aforementioned MAC header. The Tx operating modeinformation may include at least one of maximum resource unit (RU) sizeinformation, channel bandwidth information, and NSS information.

Rx operating mode information and Tx operating mode information may betransmitted in the same format. In this case, an indication bitindicating whether corresponding information is Tx operating modeinformation or Rx operating mode information may be added. In anembodiment, when a value of the indication bit is 1, it may indicatethat corresponding information is Tx operating mode. When a value of theindication bit is 0, it may indicate that corresponding information isRx operating mode.

The AP that has received the Tx operating mode information from the STAmay accept or deny a transmission operating mode change. When the APschedules the UL MU transmission of the STA, it may send a trigger framefor the UL MU transmission of the STA using the Tx operating modeinformation received from the STA. In this case, the AP may allocate anRU unit equal to or smaller than a maximum RU size which has beentransmitted by the STA and by which the AP may be accessed to the STA asthe RU unit of UL MU resources of the STA. The RU unit may be allocatedas a specific tone number unit according to an OFDMA scheme. Forexample, if the STA sends Tx operating mode information indicating thata maximum RU size by which the STA can access the AP is 52 tones, the APmay allocate UL MU resources with 26 tones or 52 tones. Furthermore, theAP may allocate the UL MU resources so that the STA can send the numberof spatial streams smaller than the number of spatial streamstransmitted by the STA.

3. Method for Sending Preferred RU Size Information and MCS Informationor TXOP Length Value in Buffer Status Report

If an STA receives a trigger frame (for random access) and makes abuffer status report, the STA may send preferred RU size (or a maximumRU size by which the STA can access an AP) information and/or preferredMCS information. The MCS information may be omitted if it is usedidentically with an MCS when a buffer status report is transmitted.Alternatively, the MCS information may indicate a maximum MCS levelwhich may be used by the STA. The STA may notify the AP of the amount ofbuffered data by sending the RU size information and the MCS levelinformation. For example, the STA may indicate the amount of buffereddata by sending transmit opportunity (TXOP) length informationdetermined based on the RU size information and the MCS levelinformation. The STA may send the TXOP length information along with theRU size information and the MCS level information or may send the TXOPlength information instead of the RU size information and the MCS levelinformation.

4. Backoff Procedure

As described above, when an STA that has transmitted Rx operating modeinformation receives a corresponding response frame, the STA may changeRx operating mode after a predetermined outage time or during an outagetime. A case where the STA sends the Rx operating mode information mayinclude a case where the STA sends its own Rx operating mode informationand a case where the STA requests Rx operating mode information about atarget STA linked to the STA. Furthermore, as described above, aresponse frame may be a frame, such as ACK/BA, M-BA, OFDMA BA or data.In this case, a transmitting STA may defer a backoff procedure during anoutage time and may perform a backoff procedure after the outage timeelapses.

5. Condition in which STA sends Rx/Tx Operating Mode Information

In order for an STA to send Tx operating mode information or Rxoperating mode information, an AP or the STA may manage a condition inwhich the transmission of Tx/Rx operating mode information by the STA istriggered. For example, if the battery of the STA is a specificthreshold value or less, the AP may trigger the condition so that theSTA sends Rx operating mode information. To this end, the AP may send aspecific threshold value as trigger information using a beacon frame,trigger frame or management frame.

In an embodiment, an AP may allow an STA to send Rx operating modeinformation only if the STA can turn off an RF chain (e.g., if the RFchain changes from 160 MHz to 80 MHz). For example, the AP may allow theSTA to send Rx/Tx operating mode information only in a specific intervalif a beacon interval is divided into an OFDMA interval and an enhanceddistributed channel access (EDCA) interval or a legacy interval and a11ax interval. For another example, if the receive signal strengthindicator (RSSI) or SNR/SNIR of a signal received from the STA is aspecific threshold value or less, the AP may send trigger information(e.g., a specific threshold value) using a beacon frame, trigger frameor management frame in order to trigger the transmission of Tx operatingmode information.

6. Method for Providing Notification of Rx Operating Mode Change Timing

FIG. 20 shows the transmission of Rx operating mode information and amethod for sending data in Rx operating mode according to an embodimentof the present invention.

Referring to FIG. 20, an STA may send Rx operating mode information inorder to change Rx operating mode into RX NSSS=2 and RX BW=40 MHz. An APmay accept the Rx operating mode information. In this case, if the APhas to send DL data for the STA, it may delay a Tx mode change for thethroughput enhancement of the DL data. If the DL data of the STA istransmitted in MU, the AP may delay a Tx mode change in order to improvean MU gain. The delay time of such a mode change may also be called anoutage time. In order to distinguish the outage time from the transitiontime of FIG. 18, the time taken to send/receive data in existing Rxoperating mode without changing Rx operating mode as in FIG. 20 may bereferred to as a delay time.

The delay time may indicate an interval in which a transmitting STA hasreceived Rx operating mode information and accepted a mode change, butthe transmitting STA sends data in existing Rx operating mode.Furthermore, the delay time may indicate an interval in which areceiving STA has sent Rx operating mode information and received aframe (ACK) indicative of the acceptance of a mode change, but thereceiving STA receives data in existing Rx operating mode. In anembodiment, the delay time may include an interval in which data istransmitted/received in operating mode prior to a change and an intervalin which a response frame (e.g., ACK) for the data istransmitted/received.

After the delay time, the AP may send a frame in mode changed based onthe Rx operating mode information received from the STA. The AP maydelay a change time according to the Rx operating mode informationtransmitted by the STA for a specific time (e.g., the delay time) usinga DL frame. In this case, the AP may notify the STA of the delay time.If not, the STA, that is, the AP, may indicate that the change timeaccording to the Rx operating mode information received from the STA isdeferred for a specific time using the DL frame. The reason for this isthat only when the AP notifies the STA of the delay time, the STA candelay an operating mode change and receive data in previous Rx operatingmode.

The AP may notify the STA of a specific interval in which the operatingmode change is deferred, and a corresponding method is as follows.

1) The AP may directly send the delay time to the STA. The AP may sendthe delay time to the STA along with information indicative of theacceptance of the Rx operating mode change. The delay time informationmay be transmitted using an MAC header, the reserved bits of aBA/ACK/M-BA/OFDMA BA frame or a specific field. The delay timeinformation may be transmitted through a data part or field newlydefined for the 802.11ax system.

2) The delay time may be previously defined. Furthermore, the AP mayindicate whether the Rx operating mode transmitted by the STA will bechanged after a predefined delay time or when a frame includinginformation indicative of the acceptance of an Rx operating mode changeis received using a DL frame. Examples of values which may be defined asthe delay time are as follows.

a. The length of current TXOP duration

b. The length of the remaining TXOP duration (in this case, an STA maychange Rx operating mode in next TXOP.)

c. Max TXOP duration length

d. Service period

e. Time left until next beacon target transmission (i.e., an STA maychange Rx operating mode in a next beacon interval.)

f. Time left until next beacon target transmission (whether the time isa beacon interval after several places may be signaled.)

The AP/non-AP STA may receive the Rx operating mode information, and maysend the delay time information while sending a frame indicative of theacceptance of the Rx operating mode information. The delay timeinformation may indicate an interval (duration) until the Rx operatingmode is changed. Alternatively, the delay time information may indicateone of the aforementioned values “a” to “f.” Alternatively, the delaytime information may indicate whether the STA will change the Rxoperating mode right after receiving a frame indicative of theacceptance of the Rx operating mode or whether the STA will change theRx operating mode after a specific time. The delay time information mayfurther include such information.

FIG. 21 shows an STA device according to an embodiment of the presentinvention.

Referring to FIG. 21, the STA device may include memory 21010, aprocessor 21020, and an RF unit 21030. Furthermore, as described above,the STA device is an HE STA device and may be an AP or a non-AP STA.

The RF unit 21030 is connected to the processor 21020 and maysend/receive a radio signal. The RF unit 21030 may up-convert data,received from the processor, into a transmission and reception band andsend a resulting signal.

The processor 21020 is connected to the RF unit 21030 and may implementthe physical layer and/or MAC layer according to the IEEE 802.11 system.The processor 21030 may be configured to perform operations according tothe various embodiments of the present invention according to thedrawings and description. Furthermore, a module for implementing theoperation of the STA according to the various embodiments of the presentinvention may be stored in the memory 21010 and executed by theprocessor 21020.

The memory 21010 is connected to the processor 21020 and stores avariety of types of information for driving the processor 21020. Thememory 21010 may be included in the processor 21020 or disposed outsidethe processor 21020 and may be connected to the processor 21020 by knownmeans.

Furthermore, the STA device may include a single antenna or multipleantennas. The contents described in connection with the variousembodiments of the present invention may be independently applied to thedetailed configuration of the STA device of FIG. 21 or two or moreembodiments may be applied to the detailed configuration of the STAdevice at the same time.

A method for transmitting and receiving data including Rx operating modeinformation of the STA device of FIG. 21 is described again inconnection with the following flowchart.

FIG. 22 shows a method for sending, by the STA device, data according toan embodiment of the present invention.

The entire description given in connection with FIGS. 16 to 20 may beapplied to FIG. 22.

An STA may receive an uplink data frame, including Rx operating modeinformation, from a first STA at step S22010. The Rx operating modeinformation is indicative of Rx operating mode to be changed by thefirst STA. The Rx operating mode information may include the number ofreceived spatial streams (NSS) information and Rx channel bandwidthinformation. Furthermore, in some embodiments, the Rx operating modeinformation may include at least one of tone number information, RUinformation, received stream number information, and MCS information.

The STA may send an ACK frame for the received uplink data frame at stepS22020. In this case, the ACK frame includes mode change acceptanceinformation. The mode change acceptance information indicates whether anRx operating mode change according to the received Rx operating modeinformation is accepted or denied. The ACK frame may be referred to asindicating all of the aforementioned BA frame, M-BA frame, and OFDMA BAframe.

If the mode change acceptance information indicates the denial of the Rxoperating mode change, the ACK frame may further include second Rxoperating mode information about the first STA. The second Rx operatingmode may be indicative of Rx operating mode different from the Rxoperating mode requested by the first STA.

For example, the uplink data frame may further include Tx operating modeinformation. In this case, only Tx/Rx operating mode ID information maybe added, and the Rx operating mode information may be used as the Txoperating mode information. The Tx operating mode information may beindicative of Tx operating mode of the STA, which is to be received bythe first STA. The Tx operating mode information may include resourceunit information indicative of a resource unit size by which the firstSTA may access the STA.

In an embodiment, the STA may further send trigger information thatinstructs the first STA to send the Rx operating mode information. Thefirst STA may send the Rx operating mode information based on thetrigger information transmitted by the STA. As described above, thetrigger information may be indicative of a specific threshold valuerelated to a battery or signal intensity.

In addition to the method of FIG. 22, the STA may send data to the firstSTA. If the STA denies the Rx operating mode information, it may sendthe data in existing Tx operating mode. If the STA accepts the Rxoperating mode information, it may send the data in changed operatingmode. More specifically, the transmission of the data in changedoperating mode may be initiated after a lapse of a delay time from thetransmission of the ACK frame. Furthermore, the aforementioned ACK framemay further include delay time information indicative of the delay time.The delay time is indicative of an interval in which data is transmittedor received in operating mode prior to a change. In an embodiment, thedelay time may further include a transition time in which mode ischanged.

Those skilled in the art will understand that the present invention maybe changed and modified in various ways without departing from thespirit or range of the present invention. Accordingly, the presentinvention is intended to include all the changes and modificationsprovided by the appended claims and equivalents thereof.

In this specification, both the apparatus and the method have beendescribed, and the descriptions of both the apparatus and method may bemutually supplemented and applied.

MODE FOR INVENTION

Various embodiments have been described in the best mode forimplementing the present invention.

INDUSTRIAL APPLICABILITY

The method for transmitting and receiving data in a wirelesscommunication system according to the embodiments of the presentinvention has been illustrated as being applied to the IEEE 802.11system, but may be applied to various wireless communication systems inaddition to the IEEE 802.11 system.

The invention claimed is:
 1. A method for transmitting, by an AccessPoint (AP), data in a wireless local area network (WLAN) system, themethod comprising: receiving an uplink (UL) frame from a station (STA),the UL frame comprising transmitting (Tx) operating mode informationindicating a Tx operating mode to be changed, wherein the Tx operatingmode information comprises a maximum number of Tx spatial streams and aTx channel bandwidth; allocating at least one frequency resource unit orat least one spatial stream to the STA based on the received Txoperating mode information, wherein the at least one frequency resourceunit is defined in units of a predetermined number of tones, and whereinthe predetermined number of tones is 26 or 52; sending a trigger framefor UL multiuser (MU) transmission, wherein the trigger frame includesresource allocation information related to at least one of the at leastone frequency resource unit or the at least one spatial stream; andreceiving a UL MU frame through the at least one frequency resourceunit.
 2. The method of claim 1, wherein the UL frame includes a mediumaccess control (MAC) header and the first Tx operation mode informationis included in the MAC header.
 3. The method of claim 1, wherein the Txoperating mode information and a reception (Rx) operation modeinformation are of a same format.
 4. An Access Point (AP) device in awireless local area network (WLAN) system, the AP device comprising: atransceiver transmitting and receiving a radio signal; and a processorcontrolling the transceiver, wherein the AP device is configured to:receive an uplink (UL) frame from a station (STA), the UL framecomprising transmitting (Tx) operating mode information indicating a Txoperating mode to be changed, wherein the Tx operating mode informationcomprises a maximum number of Tx spatial streams and a Tx channelbandwidth, allocate at least one frequency resource unit or at least onespatial stream to the STA based on the received Tx operating modeinformation, wherein the at least one frequency resource unit is definedin units of a predetermined number of tones, and wherein thepredetermined number of tones is 26 or 52; send a trigger frame for ULmultiuser (MU) transmission, wherein the trigger frame includes resourceallocation information related to at least one of the at least onefrequency resource unit or the at least one spatial stream, and receivea UL MU frame through the at least one frequency resource unit.
 5. TheAP device of claim 4, wherein the UL frame includes a medium accesscontrol (MAC) header and the first Tx operation mode information isincluded in the MAC header.
 6. The AP device of claim 4, wherein the Txoperating mode information and a reception (Rx) operation modeinformation are of a same format.
 7. A method for transmitting, by aStation (STA), data in a wireless local area network (WLAN) system, themethod comprising: transmitting an uplink (UL) frame to an Access Point(AP), the uplink frame comprising transmitting (Tx) operating modeinformation indicating a Tx operating mode to be changed, wherein the Txoperating mode information comprises a maximum number of Tx spatialstreams and a Tx channel bandwidth; and receiving a trigger frame for ULmultiuser (MU) transmission, wherein the trigger frame includes resourceallocation information related to at least one of at least one frequencyresource unit or at least one spatial stream for the STA; wherein theresource allocation information is generated based on the received Txoperating mode information, wherein the at least one frequency resourceunit is defined in units of a predetermined number of tones, and whereinthe predetermined number of tones is 26 or 52; and receiving a UL MUframe through the at least one frequency resource unit.
 8. The method ofclaim 7, wherein the UL frame includes a medium access control (MAC)header and the first Tx operation mode information is included in theMAC header.
 9. The method of claim 7, wherein the Tx operating modeinformation and a reception (Rx) operation mode information are of asame format.
 10. A Station (STA) device in a wireless local area network(WLAN) system, the STA device comprising: a transceiver transmitting andreceiving a radio signal; and a processor controlling the transceiver,wherein the STA device is configured to: transmit an uplink (UL) frameto an Access Point (AP) the UL frame comprising transmitting (Tx)operating mode information indicating a Tx operating mode to be changed,wherein the Tx operating mode information comprises a maximum number ofTx spatial streams and a Tx channel bandwidth, receive a trigger framefor UL multiuser (MU) transmission, wherein the trigger frame includesresource allocation information related to at least one of at least onefrequency resource unit or at least one spatial stream for the STA,wherein the at least one resource allocation information is generatedbased on the received Tx operating mode information, wherein thefrequency resource unit is defined in units of a predetermined number oftones, wherein the predetermined number of tones is 26 or 52, andreceive a UL MU frame through the at least one frequency resource unit.11. The STA device of claim 10, wherein the UL frame includes a mediumaccess control (MAC) header and the first Tx operation mode informationis included in the MAC header.
 12. The STA device of claim 10, whereinthe Tx operating mode information and a reception (Rx) operation modeinformation are of a same format.